Nano Spotlights

Although nanowires have attracted much interest in sensing applications, polymer single
nanowires for optical sensing, which promises greater versatility and superior performances,
remains unexplored. To date, most of these single nanowire devices have been focused
on the electrical conductance change when exposed to the species. However, optical
detection are highly desired owing to its advantages, such as fast response, immunity
to electromagnetic interference, safe operation in explosive or combustive atmosphere,
the possibility of remote monitoring through optical fibers.

Researchers in Zhejiang University, China, reported a novel approach to polymer single-nanowire
optical sensors. The compact and flexible sensing scheme demonstrated may be attractive
for very fast detection in physical, chemical, and biological applications with high
sensitivity and small footprint.

Fuxing Gu and Professor Limin Tong who leads the Nanophotonics group at the Zhejiang
University in China, together with coauthors from Department of Chemistry at Zhejiang
University, report their most recent findings in an article titled “Polymer Single-Nanowire
Optical Sensors” published online on August 2, 2008 in the journalNano Letters.

“When a weak stream of light is guided along a properly functionalized polymer nanowire,
the light output is strongly and instantly dependent on the surrounding of the nanowire,
making it ideal for high-sensitivity sensing with ultrafast response.” Prof. Tong
explained the basic idea. “Polymer single nanowires for gas optical sensing are of
great scientific interest and technological significance” Gu explains to Nanospotlight.
“To use polymer single nanowires as optical gas sensors, two major challenges must
be overcome: the fabrication of polymer single nanowires that are optically sensitive
to specific specimens, and efficiently launching light into and picking signals up
from the nanowire. First, we draw nanowires from polymer solutions doped or blended
with functional materials that act as sensitive elements. Secondly, we employ an evanescent
coupling technique for high-efficient optical launching and collection.”

“When we use a polyacrylamide (PAM) nanowire (Fig. 1) for relative humidity (RH) sensing, we got a response time as short as 30 ms, which
are over 10 times faster than those of existing RH sensors” says Gu. Polyaniline blended
with polystyrene was used to detect NO2at concentrations as low as 0.1 ppm with fast response, good reversibility, and fast
recovery time at room temperature. Bromothymol blue (a pH indicator)-doped poly(methyl
methacrylate) nanowires were used to detect NH3down to 3 ppm level with fast response.

“The remarkably fast response of the sensor can be attributed to the small diameter
and large surface-to-volume ratio of the nanowire that enable rapid diffusion or evaporation
of the water molecules, as well as fast signal collection using optical approach.”
Gu explains. The seamless combination of the functionalized polymer single nanowire
with optical scheme demonstrated here presents high versatility and flexibility for
gas sensing, and may open new opportunities for both polymer nanowires and optical
sensors. “The optical scheme extends the sensing element from conductive polymer nanowires
for electric detection to broader categories, since a majority of polymer nanowires,
no matter they are electric conductive or not, provide low optical waveguiding losses
(Fig. 2) within visible or near infrared spectral range. Moreover, compared with other materials,
polymers are much more hospitable to a variety of dopants.” says Prof. Tong. “We are
now working to use polymer single nanowires for biological sensing integrated with
microchip, and investigating optoelectronics and nonlinear effects in polymer single
nanowires for nanoscale devices.”